If a script tag is placed in the header of an HTML page, the parsing of the HTML is paused until the script is fetched, and executed. The HTML parsing only resumes once the script execution is completed. This can lead to poor user experiences. Both defer and async help avoid this. They allow parallel download of the script tag while the HTML is being parsed.
Both of these are boolean attributes with a similar syntax:
It is worth noting that the tags are only useful if the script is in the head section of the HTML. They are useless if the script is put inside the body tag.
If both are specified, precedence is given to async.
When the browser encounters a script tag with the async attribute, it downloads the script in parallel while it continues to parse the HTML. Once the script is fully downloaded, the browser pauses the HTML parsing and executes the script file. This helps improve the overall loading time of the page.
Here is a graphic to help visualize the different processes:
Most of the time defer is the preferred option because it reduces page load time the most. It does not execute until the DOM is ready, and follows the script order. So you get more control over the script’s execution as well.
Async sounds sexier but it is only useful if the script does not need the DOM or any other scripts.
And that is all you need to know about defer and async attributes on the script tag and how you can optimize the page loading time by using these. If you have any questions, feel free to drop a comment below.
Originally published at https://www.wisdomgeek.com on August 15, 2021.